Chemical apparatus and process disinfecting – deodorizing – preser – Chemical reactor – Including heat exchanger for reaction chamber or reactants...
Reexamination Certificate
1999-03-12
2001-07-03
Tran, Hien (Department: 1764)
Chemical apparatus and process disinfecting, deodorizing, preser
Chemical reactor
Including heat exchanger for reaction chamber or reactants...
C422S198000, C422S198000, C261S113000, C261S114100, C261S114500
Reexamination Certificate
active
06254840
ABSTRACT:
The invention relates to an apparatus for effecting gas/liquid contact in which the liquid forms the continuous phase. In particular; the invention relates to a reactor for preparing urea from ammonia and carbon dioxide. The invention also relates to an apparatus for the hydrolysis of urea in aqueous solutions.
Gas/liquid contact in which the liquid forms the continuous phase can be achieved in liquid-filled columns also known as bubble columns or bubble-type washers. Bubble columns may be configured in a variety of ways and may include random or structured packing materials and are often used as strippers or reactors. As described in US-A-3046307, a vertical bubble column may be is divided into a number of compartments by a series of perforated baffles, plates, or trays positioned horizontally across the column.
In bubble columns the gas phase and the liquid phase are ideally in constant intensive contact with one another throughout the entire column volume. In those bubble columns utilizing perforated trays, the gas and liquid phases are redistributed as they pass through the perforations. Although countercurrent flow may be more common, the gas phase and the liquid phase can also be passed through appropriately configured bubble columns as cocurrent or crosscurrent flows. Bubble column can be particularly useful for synthesizing urea from ammonia and carbon dioxide and for hydrolyzing urea in aqueous solutions.
A variety of bubble column configurations are possible, but common commercial designs include vertical columns about 1 to 5 meters in diameter with column heights of about 5 to 40 meters. Bubble columns used to synthesize urea from ammonia and carbon dioxide preferably have a diameter of between 1.5 and 4 meters and a height of between 10 and 35 meters.
Columns for promoting gas/liquid contact in systems in which liquid is the continuous phase are frequently provided with structures that divide the column into a plurality of compartments. To achieve this result, a column may be fitted with structured packings, trays, or a combination of structures, that define separate compartments within the column. Each of the compartments can be considered as a “continuously stirred tank reactor” (CSTR) in which the turbulent flow of the gas and/or liquid phases produces the desired stirring. In the following discussion of the present invention, reference will be made to CSTRs and compartments to describe a preferred embodiment of the present invention but such terms should be understood as descriptive rather than limiting.
The number of compartments provided in such a column is usually at least two, preferably at least five, and less than 40, preferably less than 20. The compartments are preferably formed by inserting a series of horizontal trays across the column.
If a slow reaction takes place in the liquid phase, the degree of conversion in the liquid phase can be substantially increased if the liquid phase is passed through the installation as a so-called plug flow. The contact between the gas phase and the liquid phase may result in the condensation of part of the gas phase. For example, during the preparation of urea, the heat of condensation is used for dehydrating the ammonium carbamate to form urea.
To realize a high degree of conversion, a column suitable for synthesizing urea is usually divided into a plurality of compartments lying one above the other defined by a series of horizontal trays spaced generally evenly along the length of the column as described in US-A-3046307. In this way the synthesis column is divided into a number of compartments arranged in the flow direction of the reaction mixture with the intention of providing uniform mixing of the reaction components in each of the compartments. The horizontal trays according to US-A-3046307 extend across the full width of the column and are fitted with a multitude of openings for the passage of the two-phase gas/liquid stream. However, in a column according to US-A-3046307, as gas and liquid flow through the same openings, areas of reduced flow are created that form stagnant zones within the column where no, or virtually no, flow occurs. The presence of these stagnant zones reduces the overall conversion of the reagents and degrades the column efficiency.
An alternative construction for a tray column provides for generally annular openings between the horizontal trays and the inside wall of the column. If the trays are also perforated, the fluid phase can flow through the annular openings while the gaseous phase can flow through the tray perforations. In such a configuration, however, there is a risk that the liquid phase will flow along the wall without making sufficient contact with the gas phase. This phenomenon is referred to as ‘bypassing’ and may reduce the degree of conversion achieved by the column.
Another phenomenon that has been identified as producing a reduced degree of conversion is referred to as ‘backmixing.’ Backmixing when a portion of the liquid phase flows in a direction opposite that of the intended direction of flow, for example via the perforations, as is described in US-A-4098579. In the case of ‘backmixing,’ the failure to maintain the optimum plug flow through the reactor degrades the reactor performance and reduces the yield.
An objective of the present invention is an improved apparatus for achieving improved gas/liquid contact in a system in which the liquid is the continuous phase to achieve a degree of conversion approximating the theoretically possible value.
In particular, the invention's objective is an improved reactor design suitable for preparing urea from ammonia and carbon dioxide and for the hydrolysis of urea in aqueous solutions.
A further objective of the invention is to provide trays that prevent backmixing, bypassing, and stagnant zone behavior. In particular, a further objective of the invention is a reactor design that improves the urea yield of the urea reaction and increases the throughput of the reaction components.
A further objective of the invention is to provide method suitable for coverting and modernizing existing vertical columns for both synthesis and stripping operations, particularly in urea plants. Yet a further objective of the invention is to provide a method suitable for modernizing vertically arranged hydrolyzer to decrease the effluent urea content while maintaining throughput and/or increase the reactor throughput without increasing the effluent urea content.
These objectives can be realized with an apparatus consisting of a vertical column fitted with perforated trays having an opening near the edge of the tray or between the tray and the inside column wall for liquid flow, a generally circular flange along the lower edge of the tray to create a gas cushion under the tray, a generally circular rim on the top side of the tray containing the perforations for gas flow, and a cylinder positioned between two adjacent trays that has a diameter smaller than the downward flange and larger than the diameter of the circular rim, the height of the cylinder being 40-80% of the distance between the two trays.
REFERENCES:
patent: 3779525 (1973-12-01), Tanigawa et al.
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patent: 1 542 274 (1970-03-01), None
patent: 95/31278 (1995-11-01), None
patent: 98/03477 (1998-01-01), None
Patent Abstracts of Japan vol. 2, No. 100 (C-020) Aug. 18, 1978 & JP 53 062784 A (Kobe Steel Ltd.), Jun. 5, 1978 zie samenvatting; figuren & Database WPI Section Ch, Week 7828 Derwent Publications Ltd., London, GB; Class J04, AN 78-50574 XP002085692 & JP 53 062784 (Kobe Steel Ltd.) zie samenvatting.
Patent Abstracts of Japan vol. 10, No. 185 (C-357) Jun. 27, 1986 & JP 61 035845 A (Mitsubishi Heavy Ind. Ltd.), Feb. 20, 1986 zie samenvatting; figuren & Database WPI Section Ch, Week 8614 Derwent Publications Ltd., Londaon, GB; Class J04, AN 86-090613 XP002085693 & JP 61 035845 A (Mitsubishi Heavy Ind. Co. Ltd.), Feb. 20, 1986 zie samenvatting.
Doroshenk Alexa A.
DSM N.V.
Pillsbury & Winthrop LLP
Tran Hien
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